Featured Science Paper

Marine geological constraints for the grounding-line position of the Antarctic Ice Sheet on the southern Weddell Sea shelf at the Last Glacial Maximum

This paper reconstructs the extent of the Antarctic Ice Sheet on the southern Weddell Sea continental shelf during the last ice age (Last Glacial Maximum, LGM, ca. 25,000 to 19,000 years before present). This region is of major importance for global thermohaline circulation, the “ocean conveyor”. Cool, dense water formed under large floating ice shelves in the southern Weddell Sea sinks down to the seafloor of the world’s ocean basins and spreads northwards, operating as a main driver for the ocean conveyor. The situation in the southern Weddell Sea at the LGM, however, is poorly constrained. Both ice-sheet models and sparse data from a few sediment cores published in the 1980s and 90s indicated that the ice sheet expanded during the LGM and the ice shelves grounded on the seafloor, shutting down the modern mechanism of bottom-water production. In contrast, recent reconstructions of ice-sheet thickness changes in the Antarctic hinterland of the Weddell Sea, based on surface exposure dating and computer models, concluded that ice-sheet thickening at the LGM was minor and there was no grounding of ice in deep sea-floor troughs on the continental shelf.

The new paper is a comprehensive compilation of unpublished and previously published data from nearly 100 marine sediment cores and geophysical surveys. It reviews the original interpretations of the depositional settings for the sediments and presents radiocarbon dates constraining their ages. The composition and physical properties of the sediments give evidence for widespread ice grounding on the Weddell Sea shelf in the past, with radiocarbon dates suggesting that this occurred at the LGM. In contrast to recent terrestrial and modelling studies, this new research shows that ice grounded even in the deepest parts of the shelf troughs, up to 1200 metres below modern sea level. The paper offers an explanation resolving the apparent mismatch between the conclusions from terrestrial and marine data: the LGM ice sheet on the southern Weddell Sea shelf might have had a very low surface profile, implying a very modest thickening. Taking into account the global sea-level drop of ca. 130 metres at the LGM, the ice sheet could have grounded even in the deep parts of the shelf troughs, while surface elevations in the hinterland were no different from today. This scenario is fully consistent with surface exposure dating results and corroborates their conclusions that ice-sheet build-up in the Weddell Sea sector made only a minor contribution to the global sea-level lowstand at the LGM. Moreover, the new study reveals that the processes producing dense water masses in Antarctica at the LGM must have been fundamentally different from the modern formation mechanisms.